[jdk17u-dev] RFR: 8307348: Parallelize heap walk for ObjectCount(AfterGC) JFR event collection [v2]

[Aleksey Shipilev]

> Improves ObjectCount JFR events that take considerable time at GC pause. While the event is still heavy, the parallelization helps to avoid unnecessarily large stalls. 
> 
> The backport is not clean, because there are number of contextual differences: `WorkGang`, `WithUpdatedActiveWorkers` and other renames did not happen yet. I had to massage the patch to fit 17u better.
> 
> Additional testing:
>  - [x] Linux x86_64 fastdebug `jdk/jfr serviceability/sa`
>  - [x] Linux x86_64 fastdebug `tier1 tier2 tier3`
>  - [x] Linux AArch64 fastdebug `tier1 tier2 tier3`

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains one commit:

  Backport 540c706bbcbb809ae1304aac4f2a16a5e83cb458

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Changes: https://git.openjdk.org/jdk17u-dev/pull/1441/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1441&range=01
  Stats: 41 lines in 9 files changed: 10 ins; 10 del; 21 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1441.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1441/head:pull/1441

PR: https://git.openjdk.org/jdk17u-dev/pull/1441